You're arguing over points which have nothing to do with the subject at hand. First of all the stat works for WHM and AST in the same way when they cast something that's on GCD. AST has a native small haste in Diurnal, but it doesn't affect the question of SpS in any way, since you get that whether you boost SpS or something else, so when talking about what effect the stat has in general it doesn't matter which job you're looking at. But just to humour you, the top DPS astro in Nidhogg has 238 ability usages (in a bit shorter win though) and one that's from a fight lasting over 10 minutes, they have 234 ability usages. And again I want to stress that this is all abilities, not just the ones on GCD. And funny enough, they're about the same number that WHMs have. Because all healer jobs cast roughly at the same speed. And yeah, Nidhogg is maybe one with a bit more downtime than usual, but the fact is that even with 1000 SpS you can only cast about 266 times if there's zero downtime, no mechanics to dodge and you have MP to do that constantly. Not that it matters concerning the actual subject though, but your claim that 10 minute fights have 300 casts for healers is just pulled out of thin air and about 100 casts higher than reality.
But all that is beside the point and the fact that you're arguing about it just proves further that you in fact are missing the entire point how to gauge if the stat is worthwile or not. The point is, neither the fight used as an example nor the length of the fight matter at all when the question is if SpS is more valuable compared to crit or det for healers in general. This is because, as explained in the previous post, the value of spellspeed is based on if the additional casts gained with it are worth more than the fewer casts would be boosted by crit and / or det. You will have more casts in a 15 minute fight than a 10 minute fight, but at the same time the boost from crit and det would also be bigger.
The burst phase you're using as an argument works for some DDs (and at least WAR) where damage buffs are applied. Healers don't have damage buffs and I'm at least not aware of any healer opener like this. Can you give me an example and math of a healer opener or burst which is actually better with SpS than crit or det? Because I can't think of any.
But your main argument was that the number of casts matter, and that's completely and utterly wrong. It means absolutely nothing.
I'm just gonna illustrate to you why you're wrong. Let's assume a 1 minute fight with an imaginary spell. Case 1: lots of SpS and low potency. You get 26 casts, each with potency of 100. Total potency 2600. Case 2: low SpS, high potency. You get 24 casts, each with potency of 110. Total potency 2640 and less MP used -> better.
Now obviously I pulled the numbers out of my hat and they're rigged to make crit/det better. They don't prove that SpS is factually worse, but they prove that the sheer number of casts doesn't matter in any way, which was your argument and seems to be the argument that many other people have also said previously.
The real problem at hand is that there's no good way to estimate how much crit and det actually boost the potency and this is especially hard for healers who also need to count for the healing and can't just compare raw dps numbers. SpS is only better if crit and det cannot boost the overall potency enough to overcome the smaller amount of casts but it's hard to prove this. At least before Heavensward SpS was inferior.
However for DPS classes it's pretty much a given that it's crit and det all the way unless you have a spesific need for a certain skillspeed level to get that extra ability inside some buffs, so anecdotically I'd be willing to bet that the same works for healers as well, especially because this used to be the case earlier, too. But if you have math to prove otherwise, I'll happily accept that.